Conventionally, a silicon-on-sapphire (SOS) substrate including a handle substrate made of sapphire which has high insulating properties, low dielectric loss and high thermal conductivity has been put in practical use since the 1960s until now. The SOS substrate is the earliest silicon-on-insulator (SOI) substrate, in which an SOI structure is achieved by heteroepitaxially growing silicon on the R plane (1012) of sapphire at high temperature.
In recent years, however, SOI using a SIMOX method or a bonding method has become mainstream. Accordingly, the SOS substrate is used only in devices which are not compatible with SOI whose handle substrate is made of silicon, for example, in such devices as a high-frequency device requiring low dielectric loss. In heteroepitaxial SOS, silicon is heteroepitaxially grown on sapphire different by 12% in lattice constant from silicon. It is, therefore, known that many defects due to mismatch in lattice size occur in the SOS substrate (see, for example, Non-Patent Literature 1).
In recent years, there has been a growing demand for high-frequency devices because mobile communication devices typified by cellular phones have widely spread. Therefore, the utilization of SOS substrate in this field is under consideration. However, the reality is that heteroepitaxial SOS substrate is high in defect density, thus the use thereof is limited to small discrete components (switches and the like).
Another major problem in addition to the high defect density is excessive stress applied to a silicon film. In a conventional method, a silicon film is formed at 900° C. to 1000° C. Consequently, large compressive stress occurs in silicon when a silicon film, which was grown free of stress at the time of growth, is cooled to room temperature, since a thermal expansion coefficient of sapphire is large, compared with that of silicon. In such a case, stress is proportional to a difference between growth temperature and room temperature (ΔT=875° C. to 975° C., when room temperature is 25° C.). It is pointed out that consequently, a change takes place in a conductor of silicon, and the mobility of electrons degrades to 80% or so (see, for example, Non-Patent Literature 2 and Non-Patent Literature 3). In addition, the stress of silicon grown as described above reportedly amounts to a compressive stress of 6.2×108 Pa (see, for example, Non-Patent Literature 4).